1. Field of the Invention
The present invention relates to a photoelectric conversion device having photoelectric conversion cells wherein potential at the control electrode region of the transistor is controlled through a capacitor, carriers generated upon light excitation at the control electrode are stored to control the output of the transistor by the potential generated by the stored carriers.
2. Related Background Art
FIG. 1A is a diagrammatical cross section of a photoelectric conversion device disclosed in EP Laid-Open Publication No. 132076, and FIG. 1B is an equivalent circuit of a photoelectric conversion cell of the device.
In the figures, photoelectric conversion cells are disposed on an n.sup.+ silicon substrate 1, each photoelectric conversion cell being electrically isolated from adjacent cells by an element isolation region 2 made of such as SiO.sub.2, Si.sub.3 N.sub.4 or polysilicon.
Each photoelectric conversion cell is constructed as follows.
On an n.sup.- region 3 having a low concentration of the impurity and formed using epitaxial technology or the like, a p region 4 is formed by doping p-type impurities into the n.sup.- region 3. An n.sup.+ region 5 is formed within the p region 4 using the impurity diffusion technology, or ion implanting technology. The p region 4 and N.sup.+ region 5 respectively correspond to the base and emitter of the bipolar transistor.
Formed on the n.sup.- region 3 is an oxidation layer 6 on which a capacitor electrode 7 having a predetermined area is formed. The capacitor electrode 7 confronts the p region 4, the oxidation layer 6 being interposed therebetween, and controls potential of the floating p region 4 by applying a pulse voltage thereto.
Also formed on the photoelectric conversion device are an emitter electrode 8 connected to the n.sup.+ region 5, an n.sup.+ region 11 having a high concentration of the impurity and formed on the back of the substrate 1, and a collector electrode 12 for supplying a collector potential to the bipolar transistor.
Next, the fundamental operation of the photoelectric conversion device will be described. It is assumed that the p region 4 serving as the base of the bipolar transistor has a negative potential as its initial condition. Light 13 is applied to the p region 4 to generate electron-hole pairs. Holes are stored in the p region 4 so that the potential of the p region 4 becomes positive (store operation).
In this condition, a positive pulse voltage is applied to the capacitor electrode 7 for reading light information. That is, a signal representing the received light information and corresponding to the base potential change during the store operation is outputted from the electrode 8 of the floating emitter (read operation). In this case, a non-destructive read operation is possible since the stored charge in the base or p region 4 is scarcely reduced.
To remove holes stored in the p region 4, the emitter electrode is grounded and the capacitor electrode 7 is applied with a positive refresh pulse voltage. Upon application of this pulse, the p region 4 is forward biased relative to the n.sup.+ region 5 to accordingly remove stored holes. After the refresh pulse falls, the p region 4 resumes the initial condition of negative potential (refresh operation). Thereafter, similar store, read and refresh operations are repeated.
In summary, according to the above method, carriers generated by incident light are stored in the base or p region 4, and current passing between the emitter electrode 8 and the collector electrode 12 is controlled by the quantity of stored charges. Since stored carriers are amplified by the amplification function of the cell itself, a large output, high sensitivity and low noise can be realized.
Base potential Vp generated by light-excited carriers of the base is given by Q/C, where Q is the charge quantity of carriers stored in the base, and C is the capacitance of the base. As the cell size becomes small due to high integration density, the values of Q and C become small. Thus, as is apparent from the formula, the potential Vp generated by light excitation remains substantially constant. Therefore, it can be said that the above-proposed method is advantageous for obtaining a high resolution of the device.
With such a photoelectric conversion device, only light information stored in a photoelectric conversion cell is read from the emitter electrode. Therefore, it is necessary to measure the quantity of light or detect a peak value prior to a read operation, resulting in an obstacle to high speed operation. Further, there arises a problem that the circuit for regulating exposure time and gain based on the detected value becomes complicated in its construction and operation.